chris,

i thk the JFK turnaround should be ok.
it is the same airplane. it's got to park there for sometime, esp these days (to get disinfected).
US custom is not that bad (though JFK is not a good port ot enter US).

i would be more concerned on the YVR turnaround because they are different planes. (but it seems that you have much time to spend).

also, i think there is a reason to go JFK-YVR-DFW (vs JFK-SFO-DFW), it is CX plane/FA vs AA plane/FA!

btw, if you need to get some good sleep in YVR, the fairmont at the airport is very good. CX put me there once.